1. Field of the Invention
The invention relates to an epitaxial substrate and fabrication thereof, and more in particular, to an epitaxial substrate having a nano-rugged and non-patterned epitaxial surface and fabrication thereof.
2. Description of the Prior Art
Compound semiconductor materials, such as GaN, AlGaN, AlInGaN, and other III-V group compounds, or CdTe, ZnO, ZnS, and other II-VI group compounds, have been used for a wide variety of substrates of microelectronic devices including transistors, field emission devices, and optoelectronic devices, but not limiting the above described.
Taking a GaN-based microelectronic device as an example, a major problem in manufacture is that the GaN semiconductor layer manufactured must have low defect density to ensure the performance of the GaN-based microelectronic device. It is understood that one of these contributors for defects is the lattice mismatch between the substrate and the GaN layers grown on the substrate. Therefore, though the GaN layer has been grown on the sapphire substrate, but it is well known that the GaN layer is preferably grown on the AlN buffer layer previously formed on the SiC substrate to reduce the defect density, especially to reduce the density of threading dislocations. Even though there are these considerable progresses, it is still the goal desired to reach to reduce the defect density continuously on the research.
It is also well-known that the condition of epitaxy is controlled to achieve the lateral epitaxy by use of the substrate with patterned surface, which benefits in preferred orientation of epitaxy, to reduce the defect density or control defects. For example, a GaN semiconductor layer can be formed on the sapphire substrate with patterned surface in lateral epitaxial way to control dislocations in extending laterally to reduce the density of threading dislocations.
However, all of the prior arts regarding manufacture of the epitaxial substrate with patterned surface must utilize a photolithography process. Obviously, the prior arts regarding manufacture of the epitaxial substrate with patterned surface have high manufacture cost and slow production speed.